Q:Will eating a calorie of fat make you fatter than eating a calorie of carbohydrate?
A: From many kinds of studies conducted over years, we are quite confident now that a calorie from fat will cause a similar amount of weight gain as a calorie from carbohydrate. There are some interesting questions about whether eating carbohydrate calories versus fat calories will make you eat more calories, but based on what you put into your mouth, it's pretty clear that the source of the calories is really not important.
[Whether fats or carbohydrates are more filling] is one issue that's been raised — but it's been raised on both sides. The best way to get to the bottom line is to look at long-term studies where we randomize people to a high-fat/low-carb diet or to a low-fat/high-carb diet and follow them for at least a year or more. That kind of study takes into account the possibility that one kind of diet provides more satiety; so, over the long run you would see more weight loss on that diet. But those studies — half a dozen or more have been done — show quite clearly that the percentage of calories from fat has very little effect on long-term weight loss.
One possible footnote to this issue relates to some recent evidence on trans fats. We have seen in our studies that people who eat more trans fats seem to gain more weight, even when the total calories are the same. I was a little skeptical about that, in part because we're not quite sure we can measure calorie [intake] precisely enough. [It's hard for people to track their portion sizes to the gram, or even be sure of exactly what they're eating, especially if they ever eat out.] But in recent five-year feeding study in monkeys — they're animals so you can control their diets — the monkeys on the high-trans-fat diet gained more weight. They gained about 7% of their body weight over a five-year period, compared to the monkeys on a low-trans-fat diet, who gained about 1.5% of their body weight over five years.
So there may be something more complicated going on there. But there's not any good data [to explain why a calorie of trans fat should cause more weight gain than a calorie of something else]. It may be that on the high-trans-fat diet you're more likely to push those calories into your fat cells rather than your muscle cells — and muscles burn calories 24 hours a day. In the long run, that could make a difference in weight gain. But that's speculation. We're really not sure.
We've now looked at over 250,000 men and women for up to 30 years, and we [also] haven't seen that the percentage of calories from fat or from carbohydrates in your diet makes any difference in relation to heart attacks, various cancers or stroke. Having said that, the type of fat is very important, and so is the type of carbohydrate. So we find that trans fats, again, are particularly harmful with regard to type 2 diabetes and heart disease. On the other hand, unsaturated fats are actually beneficial in terms of reducing the risk of heart disease and type 2 diabetes. It's the same with carbohydrates. The total amount is not important. But high intake of refined starch and sugar is related to a higher risk of heart disease and diabetes, whereas high-fiber whole-grain carbohydrates are related to a lower risk. That's not too surprising, as we know that high intakes of sugar and refined starch have an adverse effect on blood glucose levels.
So the quality of the diet is really important, but just looking at fat versus carbohydrate misses where all the action is.